Chuckles-
"How exactly would they put a lien on a boat that doesn't have a state registration on it,"
Who needs a lien? Under admiralty law, you physically "arrest the boat" and literally nail the lien notice to the main mast while "arresting" the boat by physically chaining it to the dock.
One hopes that they'd use scotch tape instead of a nail these days, but, you know, tradition!